Basalt at Joaquin Miller Park

Oakland has a long narrow strip of basalt mapped along the Hayward fault between Park Boulevard and Seminary Avenue, but it’s hard to find outcrops of it. I remember searching for it in the valley south of the Mormon temple and coming up empty. There’s supposed to be a bit of it in the hill at 98th Avenue that deflects Arroyo Viejo, though that’s so shattered that it’s hard to tell what you’re looking at. But in Joaquin Miller Park there’s a second tendril of the basalt running from under Joaquin Miller High School up the creek bed. There’s a nice exposure along the road next to the Browning monument, where this chunk sits.

joaquin miller basalt

The bluish color is correct; like the serpentinite around it, this rock is of Jurassic age and is part of the Coast Range Ophiolite, and a long history of burial, compression, upheaval and tectonic motions has left it rather altered from its original looks. But once it was a thick flow of oceanic lava. In fact, I may be fooling myself in seeing the vague remains of lava pillows in this roadcut:

joaquin miller basalt

I didn’t have the time (or the magnifier) to examine this closely—just another question to follow up on some time.

At some point in the next few days, I’ll be finalizing the contents of a new interpretive sign that will be installed at the park. Along with the usual subjects of plants and animals and human history, the rocks will get their due.

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One Response to “Basalt at Joaquin Miller Park”

  1. Dan L Says:

    I think you’re right about fooling yourself
    about seeing a lava pillow.
    Lava is never soft enough for that :-)

    Maybe spheroidal weathering?

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